Many thanks for your thoughtful response.
 
A specific case prompting my question relates to Turbonnillas collected
via hand dredge from estuarian waters.  Many live collected species are
very translucent and remain so many years after collection (dessicated in
alcohol, thoroughtly dried, sealed in air tight containers, and stored
away from light).  Dead collected specimens are invariably opaque, but do
not necessarily exhibit tiny scratches of wear under 20X magnification.
None of these are beach collected.  However, I'm sure some of the
tumbling you describe does occur, but not much and not in the kind of
sand you describe since these estuarian bottoms are mostly a soft mud and
fine sand mixture covered with seagrasses.  They are also not subject to
much wave action and I imagine sink into the bottom fairly quickly after
death.  The "dull, opaque, and apparently white" appearance of these dead
shells is exactly the descriptive appearance given by many authors to
numerous micro shells like Turbonillas described in the literature when
only dead collected specimens are what they have available; and, most
often are ocean collected.
 
Marlo
 
I like your "acid etching" suggestion better
 
Paul Monfils wrote:
 
> Hello Marlo,
> I have no proven answer to your query, but have certainly observed
> the same phenomenon, and thought about it some.  All I can offer is
> my unsubstantiated theory.  We do know of course that it is not
> merely a time-related change, because live-taken specimens with good
> transparency will retain their transparency for many years.  So, it
> is apparently some external and fairly vigorous agent that causes the
> rapid transformation from transparent to opaque white.  When you say
> "dead collected", you presumably mean "beach collected", probably in
> the "beach drift" or "shell hash" that is deposited near the high
> tide line - and that, I believe, is the key.  A shell doesn't just
> ride in on a wave and get gently deposited in the drift line.  Before
> it ever reaches that final resting place, it has been driven up and
> down the beach many times by the ebb and flow of the surf, each time
> tumbling along among millions of rapidly moving sand grains.  Sand
> grains (in most localities) are largely quartz and other dense
> minerals, far harder material than the calcium salts of the shell.
> Consequently, the shell gets innumerable tiny (probably mostly
> microscopic) scratches all over it.  The result (I theorize) is the
> same as you would get if you sandpapered a sheet of plexiglass, or
> even a pane of glass - a change from glossy and transparent to dull,
> opaque, and apparently white.  "Frosted glass" is made by two
> separate processes - acid etching and/or sand blasting.  I suspect
> that what we have here are "frosted shells" resulting from sand
> blasting.
> Hmm - that raises an interesting question - in areas where the sand
> is mostly of coral origin, are the microshells generally in better
> condition than in areas where the sand is primarily of rock origin?
> Anybody know?
>
> Paul M.